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Lean and Mean

Teach your students to lean the mixture

How many students or new private pilots can properly lean an aircraft engine in cruise flight? There is some evidence that proper leaning skills are not being taught, or perhaps students are not learning.

Most primary flight training is done near the airport at relatively low altitudes. Maneuvers and pattern work make up most of the private pilot curriculum. New pilots typically get very little instruction in leaning, even though it is an important cross-country skill.

There is a fear of the mixture control, frequently enforced by flight school procedures that say there should be no leaning below 5,000 feet. Too much fuel or not enough? Either way won't produce an optimal outcome. Frequently, we hear of pilots running out of gas because they failed to lean and thus burned far more fuel than anticipated. The following accident shows that it can also go the other way.

A student pilot on his third supervised solo cross-country flight experienced a rough-running engine. Oil soon covered most of the front windscreen. The engine lost power, the student made a forced landing in a clearing, and the airplane struck a tree and a building. On examination of the wreckage, the investigator observed a two-inch diameter hole in the engine's upper case near the number two cylinder. The connecting rod to the number two piston was disconnected from the piston due to a failure of the piston skirt in the piston pin area. The rings on all pistons appeared to have been sticking, with over-temperature signatures noted on all the intake and exhaust valves. The tops of the piston were abnormally clean. The engine had a total time of 3,794 hours and 1,987 hours since the last major overhaul. Normal overhaul time is typically 2,000 hours, so it was a high-time engine.

The probable cause for the failure was the pilot's use of an excessively lean mixture, which led to engine overheating, detonation, subsequent failure of the number two piston, and a forced landing.

Demonstrate proper leaning to your students periodically-not just once or twice. Then ask them to perform the process regularly. It should be part of the cruise checklist. It's a good idea to teach both methods of leaning-using an exhaust gas temperature gauge (if the airplane is so equipped); and also leaning to engine roughness or rpm peak, and then enriching per the manufacturer's recommendation. A student who is familiar with both methods is well equipped to handle a variety of airplanes.

Bruce Landsberg is executive director of the AOPA Air Safety Foundation.